1. Field of the Invention
This invention relates to a power amplifier circuit for use in, for example, audio apparatuses or audio visual (AV) apparatuses.
2. Description of the Prior Art
Audio apparatuses and audio visual (AV) apparatuses include power amplifiers. There is a power amplifier circuit having a power amplifier and a known system for reducing a power loss and suppressing the generation of heat in the power amplifier.
The known system in the power amplifier circuit includes a high-voltage power supply and a low-voltage power supply. The known system selects one from the high-voltage power supply and the low-voltage power supply in response to the voltage of an output signal from the power amplifier. The known system uses the selected power supply to feed electric power to the power amplifier. When the voltage of the output signal from the power amplifier exceeds a threshold, the high-voltage power supply is used. Otherwise, the low-voltage power supply is used. Thus, the known system implements switching between the high-voltage power supply and the low-voltage power supply in response to the voltage of the output signal from the power amplifier.
In the case where the output signal from the power amplifier has a small amplitude so that the low-voltage power supply is used, the generation of heat is more effectively suppressed as the voltage of the low-voltage power supply is decreased. In the known system, the switching between the high-voltage power supply and the low-voltage power supply tends to be wrong when the voltage of the low-voltage power supply is set to a relatively small level.
The voltage of the output signal from the power amplifier varies in accordance with the impedance of a loudspeaker connected thereto. Thus, conditions of the comparison between the output signal voltage and the threshold depend on the impedance of a loudspeaker connected to the power amplifier. In the known system, the switching between the high-voltage power supply and the low-voltage power supply tends to be wrong when a loudspeaker having a relatively low impedance is connected to the power amplifier.